As technology progresses and electronic devices become more advanced, one issue that continues to plague system designers is the inevitable signal loss over a span of cable—this power loss is referred to as attenuation. Attenuation is a key specification for all cables (e.g., coaxial cable, a type of cable that supplies a signal to a cable modem) and affects the propagation of signals in upstream and downstream electrical circuits. This is a considerable issue in the cable and telecom industries, which run thousands of miles of cable to provide services to their users. The basic function of a coax cable is to act as a pipe, transferring RF (radio frequency) signals from a signal source to a receiver (e.g., from the cable plant to the end-user). In a perfect world, the amount of power exiting the coax cable should be equal to the amount that entered it. In the real world, however, this is not the case, and some power is lost along the length of the RF cable, causing less power to reach the user than originally entered the RF cable. Cable and telecom companies typically use a number of tactics to manage this problem. These tactics may include placing amplifiers along the line and using lower-loss cable. It is well known that attenuation varies depending on the type of cable and is usually directly correlated to the length of a specific cable. Unfortunately, because lower-loss cable is typically more expensive, cable and telecom companies are reluctant to use it. Table 1 provides an overview and compares the various coax-cable signal losses.
TABLE 1Approximate Coax Cable Signal Loss (Attenuation) in dB per 100 ftRG-174RG-58RG-8XRG-213RG-6RG-11RF-9914RF-9913 1 MHz 1.9 dB 0.4 dB 0.5 dB0.2 dB0.2 dB0.2 dB0.3 dB0.2 dB 10 MHz 3.3 dB 1.4 dB 1.0 dB0.6 dB0.6 dB0.4 dB0.5 dB0.4 dB 50 MHz 6.6 dB 3.3 dB 2.5 dB1.6 dB1.4 dB1.0 dB1.1 dB0.9 dB100 MHz 8.9 dB 4.9 dB 3.6 dB2.2 dB2.0 dB1.6 dB1.5 dB1.4 dB200 MHz11.9 dB 7.3 dB 5.4 dB3.3 dB2.8 dB2.3 dB2.0 dB1.8 dB400 MHz17.3 dB11.2 dB 7.9 dB4.8 dB4.3 dB3.5 dB2.9 dB2.6 dB700 MHz26.0 dB16.9 dB11.0 dB6.6 dB5.6 dB4.7 dB3.8 dB3.6 dB900 MHz27.9 dB20.1 dB12.6 dB7.7 dB6.0 dB5.4 dB4.9 dB4.2 dB 1 GHz32.0 dB21.5 dB13.5 dB8.3 dB6.1 dB5.6 dB5.3 dB4.5 dBImpedance (Z)  50 Ω  50 Ω  50 Ω 50 Ω 75 Ω 75 Ω 50 Ω 50 Ω
Attenuation is defined in terms of decibels per unit length at a given frequency (the longer the cable, the greater the loss) where the loss is also frequency-dependent, typically increasing with frequency. Other factors, however, may also impact a cable's attenuation value. For example, at frequencies of 1 GHz, an RF cable normally exhibiting a loss of 10 dB may experience a loss increase of 1 dB or more when physically bent. Other factors, including the temperature and weather, can also have an impact on the loss. In general, 75Ω coax cable is used almost exclusively for TV and VHF FM applications. But for commercial, amateur and CB applications, 50Ω coax cable has been adopted as the standard.
There are a number of causes for the above-described power loss. A first cause of power loss is radiated loss. Radiated loss is generally the least important cause because only a miniscule amount of power is generally radiated from most cables. Nevertheless, very inexpensive coax cables may have a very poor outer braid, and in this case, radiated loss may represent a noticeable element of the loss. As discussed in the following paragraphs, most loss can be attributed to the resistive and dielectric losses within the coax cable.
A second cause of loss is resistive loss within the coax cable. Resistive loss arises from the basic resistance of the conductors (e.g., the copper wire within the cable)—the current flowing in the conductors results in heat being dissipated. The actual area through which the current flows in the conductor is limited by the skin effect, which becomes progressively more apparent as the frequency rises. To decrease the power loss due to resistance, multi-stranded conductors can be used, as they have a lower resistance than solid conductors. To reduce the level of power loss in the coax cable, the conductive area must be increased, resulting in lower-loss coax cables being larger (and heavier) than higher loss cables. The resistive losses may also increase as the square root of the frequency, meaning that resistive losses normally dominate at lower frequencies.
A third cause of loss is dielectric loss. The dielectric loss represents a major loss in most coax cables. As with resistive losses, the power lost as dielectric loss is dissipated as heat. The dielectric loss is typically independent of the size of the RF cable, but increases linearly with frequency. Therefore, where resistive losses increase as the square root of the frequency, dielectric losses increase linearly, causing the dielectric losses to dominate at higher frequencies.
To combat such power loss, cable and/or telecom companies typically install power amplifiers, spaced throughout the cable network (the “cable plant”). Such amplifiers are adjusted when the cable plant is installed, and again every time an additional tap is installed or removed from that cable plant. Furthermore, the cable installers typically over-power each cable strand (a “hot” cable), somewhat, to ensure sufficient signal strength at each tap. At each tap, the power is usually attenuated to proper levels to avoid damaging downstream circuitry. This attenuation is accomplished by either installing a physical attenuator at each tap and/or coiling extra lengths of cable until the correct power level is achieved. This strategy is problematic because the attenuators must be changed and/or the cable re-coiled every time power changes are propagated through the cable plant.
Currently, the cable companies set the downstream levels using a tap box. The tap box, which may or may not be power-amplified, is usually located at the service line (e.g., the distribution line carrying a signal from a cable plant) in the street or alley and provides a connection to individual users. The tap box acts essentially as a “T-connection” where the service line carrying the high power signal (e.g., ˜20 to 52 dBmV) from the cable plant may proceed to the next house while the power of the signal being diverted to each user is dropped until a desired target downstream level is reached. This type of tap box is well known in the industry and typically sets only the downstream power, tapping out a small portion of the power and feeding it to the user. See, for example, U.S. Pat. No. 3,989,333 to Jack Cauldwell and U.S. Pat. No. 4,691,976 to Judith A. Cowen. Cauldwell and Cowen both teach cable tap connectors which divert a signal from a cable service line to the user without disrupting the service line's flow to the next user. In some instances, cable companies may insert a physical attenuator plugs into a tap box.
Because the RF signal usually travels through a great distance of cable (e.g., 150-200 feet) from the service line to the user, where standard RF cable can yield more loss at high frequency than at low frequency, there may be much downstream attenuation but very little upstream attenuation. The present disclosure is designed to take this large disproportion between downstream and upstream attenuation into account. For example, the upstream may have a low frequency range of about 5 to 100 MHz while the downstream may range from about 100 up to about 800 MHz. Currently, at least in the United States, typical DOCSIS modems are capable of transmitting upstream signals in the frequency band of about 5 to 54 MHz (however, this range may be expanded upwards), while capable of receiving cable channels or data channels in the range of about 88 to 750MHz.
Therefore, what is needed is an effective strategy to deal with cable power fluctuations at the tap, which avoids the necessity of service visits by cable/telecom workers.
In addition to the above, it is important to note that cable modem chips, such as those made by Texas Instruments or Broadcom, are designed to be optimized specifically for the above defined installations. Cable companies work closely with chip manufactures to define the operating ranges of these devices that attach directly to the cable plant, and as such, must meet strict requirements for out-of-band noise and in-band noise. These specifications are detailed in “Data Over Cable System Interface Specification” by CableLabs and cover the PHY(sical), MAC (Media Access Control), DLC (Data Link Control), Networking protocol layers and other aspects of the cable modem operation on the plant. It is sufficient to state that the specifications have been optimized to address the operational requirements of the hundreds of millions of cable modems which are installed and operate throughout the world.
These cable modems have been optimized to address the operational requirements of the home user, and as previously discussed, those requirements address locations where most cable modems are located behind a significant amount of downstream attenuation due to the 150-200 feet of high-loss cable connecting the plant to each home.
However, for equipment located directly on the plant, both the levels and the actual operational requirements are different from those of the millions of home cable modems. More specifically, and in addition to the downstream levels being higher, the cable modems installed directly attached to the plant must accept very high (typically as high as +35 dBmV, but possibly as high as +45 dBmV) input signals. Such high levels require significant downstream attenuation, which has been described above. However, not yet described, is that these cable modems are required to inject minimal levels of spurious emissions into the plant. All cable modems employ output level control, which is defined in the DOCSIS (Date Over Cable System Interface) Specification to be +8 dBmV to +54 dBmV (some devices may be slightly higher or lower). Such a wide range in output level is achieved by a DAC (Digital to Analog Converter), usually a 14 bit DAC, in the cable modem which performs level adjustment to the upstream signal. The CMTS (Cable Modem Termination System), located in the cable operators' facilities, controls the output level of the cable modem, and will adjust the cable modem upstream level to arrive at an acceptable input level.
Most home-installed cable modems have a significant amount of downstream attenuation due to the 150-200 foot cable run, and as a result, the CMTS does not need to adjust the upstream level to be very low. For example, if 200 feet of RG58 is installed, then the downstream will have been attenuated by at least 10 dBi once connector losses are included. This means that the CMTS will set the upstream signal level to be a value—for example +35 dBmV—so that the upstream signal falls within the desired range at the CMTS. However, if the same cable modem is installed directly connected to the plant, then the downstream will be 10 dB higher, and when adjusted with a fixed attenuator as is often the case, the CMTS will set the upstream level to be 10 dB lower, or in this case, +25 dBmV which is within the operational range of the cable modem.
Plant engineers, the technical team that manages the levels on the core infrastructure of the plant, have important rules regarding what levels are allowed and not allowed. They have learned that cable modems should never transmit at low levels, since their inherent signal-to-noise (SNR) levels are higher when they transmit at +8 dBmV rather than at +48 dBmV. This is understandable, since SNR is a function of the number of bits in the DAC and with each bit providing 6 dB of gain, the +8 dBmV signal will have 5 bits employed for signal level shifting, reducing the SNR. Plant engineers require a minimum level between +45 dBmV and +52 dBmV, maximizing the cable modem DAC level and minimizing the noise injected onto the plant. They rely on separation of external filters—employing fixed downstream filters and fixed upstream specific filters to attempt to achieve the desired levels. The static attenuation filters are installed and do not change, thus allowing the CMTS to automatically adjust the upstream levels to achieve the desired range.
This invention, “CABLE MODEM WITH DUAL AUTOMATIC ATTENUATION,” provides control algorithms that enable the cable modem to operate with maximum SNR while addressing the concurrent level adjustments of the CMTS. This invention automatically adjusts the downstream and upstream attenuation levels independently, and in the presence of automatics adjustments from the CMTS to control the upstream levels. The present application achieves optimal downstream levels, so that the BER is minimized, and concurrently adjusts the upstream levels to enable the cable modem to operate at maximum SNR, while ensuring that the absolute levels of the cable modem are within the defined tolerances defined, not by the CableLabs DOCSIS standards, but by the experience of the Plant Engineers.